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Stress seeds provide an interesting example of
the processes considered above by which scalar, vector, and
tensor metric perturbations are generated and affect
the temperature and polarization of the CMB. They are also the
means by which cosmological defect models form structure
in the universe. As part of the class of isocurvature models,
fluctuations, including the (scalar) curvature perturbation
are absent in the initial conditions.
To explore the basic properties
of these processes, we employ simple examples
of stress seeds under the restrictions they are causal and scale
with the horizon length. Realistic
defect models may be constructed by superimposing such
simple sources in principle.
We begin by discussing the form of the stress seeds themselves
and then trace the processes by which they form metric perturbations
and hence CMB anisotropies (§VC).